Process for preparing alkyl hydroxyalkyl cellulose ethers

ABSTRACT

The process for preparing an alkyl hydroxyalkyl cellulose ether comprises the steps of: a) alkalizing cellulose to obtain alkali cellulose, b) reacting alkali cellulose with an alkyl halogenide until an alkyl cellulose ether with a D.S.(alkyl) value of from 0.7 to 2.5. is attained, c) removing excess alkyl halogenide from the reaction mixture, and d) reacting the alkyl cellulose ether with an alkylene oxide in a liquid suspending agent to produce an alkyl hydroxyalkyl cellulose ether. An alkyl hydroxyalkyl cellulose ether is produced which has an D.S.(alkyl) of from 0.7 to 2.5 and an MS (hydroxyalkyl) from 0.03 to 2.0, wherein not more that 15 percent of the hydroxyalkyl groups are capped with an alkyl group.

This application claims the benefit of Provisional Application No.60/292,496, filed May 21, 2001.

The present invention relates to new alkyl hydroxyalkyl cellulose ethersand a process for preparing them.

BACKGROUND OF THE INVENTION

Alkyl hydroxyalkyl cellulose ethers and processes for preparing them aregenerally known in the art. The alkyl hydroxyalkyl cellulose ethers finduse in a large variety of technology areas including the building,pharmaceutical and food industry and in a wide range of applications,for example in pharmaceutical capsules and tablets, or as suspendingagents in vinyl chloride polymerization. Depending on the type ofend-use, the alkyl hydroxyalkyl cellulose ethers vary in molecularweight, type or degree of alkyl and hydroxyalkyl groups present in thecellulose ether or combinations thereof.

A commonly known process for preparing alkyl hydroxyalkyl celluloseethers includes the steps of alkalizing cellulose and etherification ofthe alkali cellulose with an alkylene oxide and an alkyl halogenide. Theetherification with an alkylene oxide requires the presence of acatalytic, that means sub-stoichiometric, amount of a base, whereas theetherification with an alkyl halogenide consumes a stoichiometric amountof a base.

GB-A-1,003,662 discloses a process for preparing alkyl hydroxyalkylcellulose ethers wherein cellulose is pre-treated with an alkali halidein the presence of alkali and water until a D.S. value of 0.05 to 0.5 isattained, the amount of alkali contained in the pre-treated cellulose isreduced to less than 10 percent, based on the pre-treated cellulose andthe pretreated cellulose is contacted with a gaseous alkylene oxide inthe presence of the reduced amount of alkali until an M.S. value of morethan 1.0 is attained. Pre-treatment can be conducted in the liquidphase, but that it is more conveniently conducted in the gaseous phase.GB-A-1,003,662 teaches that the reaction of the pre-treated celluloseether with the alkylene oxide can be conducted in a methanol-acetonemixture, but for achieving an M.S. of more than 1 and a good solubilityof the resulting cellulose ether, the pre-treated cellulose ether shouldbe reacted with gaseous alkylene oxide. Unfortunately, the producedcellulose ethers are hot-water-soluble or thermoplastic and thus are notvery useful for many important applications. Furthermore, as taught inGB-A-1, 003,662, the pre-treated cellulose should be contacted with agaseous alkylene oxide for etherification. If the etherification withpropylene oxide is carried out in liquid phase in the presence of amixture of methanol and acetone, as disclosed in the comparative processin Example 1, propylene oxide will react with methanol to produce, forexample, methoxypropanol or 1,2-dimethoxypropan as undesirableby-products. Moreover, any further purification, which is necessary formany applications, of the thermoplastic or hot-water-soluble celluloseether produced in liquid phase by washing with hot water presents majordifficulties. Moreover, the produced alkyl hydroxyalkyl cellulose etherswith a D.S. value of only from 0.05 to 0.5 are not useful for manyapplications in the building and other industries.

German Offenlegungsschrift DE-A-33 16 124 (corresponding to U.S. Pat.No. 4,550,161) teaches that the process disclosed in GB-A-1,003,662 hassubstantial disadvantages. It teaches that the absence of liquiddispersants leads to inhomogeneously etherified products and that thecellulose which has been pre-treated with an alkali halide has to bepurified prior to the reaction with an alkylene oxide because of thehigh content of residual alkali. To overcome these disadvantages, GermanOffenlegungsschrift DE-A-33 16 124 discloses a process with thefollowing steps: (a) alkalizing the cellulose ether, (b) etherifying thealkali cellulose with ethylene oxide or propylene oxide in the presenceof a sub-stoichiometric quantity of a base, (c) increasing the quantityof the base and (d) etherifying the cellulose ether with methylchloride. In all steps water is present. In at least one stepdimethoxyethane, an alkanol, an alkane diol and/or an alkoxyalkanol isused as a dispersing auxiliary.

In U.S. Pat. No. 4,650,863 a similar process is disclosed except thatthe dispersing auxiliary is dimethyl ether, which is used either aloneor in mixture with dimethoxyethane, an alkanol, an alkane diol and/or analkoxyalkanol.

Unfortunately, in the processes taught in U.S. Pat. Nos. 4,550,161 and4,650,863 the starting cellulose pulp is insufficiently activated by thelow quantity of the used caustic leaving too many crystalline areas ofthe cellulose fibers intact.

In other known, commonly used processes the etherification of the alkalicellulose with an alkylene oxide and an alkyl halogenide are conductedsimultaneously.

According to the procedure described in U.S. Pat. Nos. 4,550,161 and4,650,863 the free hydroxy groups of the anhydroglucose units in thecellulose react with an alkylene oxide, which means that they areetherified with hydroxyalkyl groups. The hydroxyalkyl substituents againcontain a free hydroxy group. Therefore, there are different kinds offree hydroxy groups available after hydroxyalkylation. There are freehydroxy groups in the anhydroglucose and in the hydroxyalkyl groups aswell. Depending on the employed amount of alkyl halogenide for thealkylation, both kinds of the free hydroxy groups are etherified withalkyl groups. In case of the etherification of the hydroxyalkyl groupsthe common term is capping. It is difficult or very circumstantial tocontrol which hydroxy groups are capped with alkyl groups.

In view of the deficiencies of the prior art processes, it would stillbe desirable to provide a new process for producing alkyl hydroxyalkylcellulose ethers. For enriching the art, it would particularly bedesirable to provide new alkyl hydroxyalkyl cellulose ethers.

SUMMARY OF THE INVENTION

One aspect of the present invention is a process for preparing an alkylhydroxyalkyl cellulose ether which comprises the steps of:

-   a) alkalizing cellulose to obtain alkali cellulose,-   b) reacting alkali cellulose with an alkyl halogenide until an alkyl    cellulose ether with a D.S.(alkyl) value of from 0.7 to 2.5 is    attained,-   c) removing excess alkyl halogenide from the reaction mixture, and-   d) reacting the alkyl cellulose ether with an alkylene oxide in a    liquid suspending agent to produce an alkyl hydroxyalkyl cellulose    ether.

Another aspect of the present invention is an alkyl hydroxyalkylcellulose ether having a D.S.(alkyl) of from 0.7 to 2.5 and an M.S.(hydroxyalkyl) of from 0.03 to 2.0, wherein not more than 15 percent ofthe hydroxyalkyl groups are capped with an alkyl group.

DETAILED DESCRIPTION OF THE INVENTION

Preferably, all reaction steps in the process described below arecarried out while the reaction mixture is agitated. The reaction stepscan be conducted using one or several of the apparatuses known fromcellulose ether chemistry, for example kneaders, agitated vessels,agitated reactor-mixers, or screw mixers. The reaction mixtures andreaction apparatuses may be rinsed with an inert gas, such as nitrogen,to remove oxygen.

Reaction step a) can be carried out in a known manner. Generally finelydivided, preferably ground, cellulose is mixed with water and an alkalimetal hydroxide, preferably sodium hydroxide. The cellulose employed iseither of natural origin, for example cotton linters or wood pulp, or itis in a regenerated form, such as cellulose hydrate. The averageparticle size of the cellulose is preferably less than 2.5 mm, morepreferably less than 1 mm, most preferably less than 0,5 mm. The alkalimetal hydroxide can be used in a solid form or in the form of an aqueoussolution. When an aqueous solution is used, its alkali metal hydroxideconcentration generally is from 15 to 70 percent, preferably from 25 to60 percent, most preferably from 45 to 55 percent, based on the totalweight of the aqueous solution. Generally the molar ratio between thealkali metal hydroxide and the cellulose in the alkalization step,calculated on the basis of an anhydro-D-glucose unit, is from 0.5 to10:1, preferably from 1.0 to 6.0:1. The alkalization reaction can becarried out in the presence of a liquid suspending agent as describedfor the reaction step b) below. Preferably, the weight ratio between theliquid suspending agent and the cellulose is from 0.1 to 10.0:1, morepreferably from 0.2 to 5.0:1. Preferably the weight ratio between thewater and the liquid suspending agent, if both are present, is from 0.15to 2.5:1, more preferably from 0.3 to 1.25:1. The reaction between thecellulose and the alkali metal hydroxide is generally carried out at atemperature of from 10 to 50° C., preferably from 20 to 45° C., and at apressure of from 10 to 1,000 kPa, preferably from 100 to 800 kPa.

In the reaction step b) the alkali cellulose is reacted with an alkylhalogenide until an alkyl cellulose ether with a D.S. (alkyl) value offrom about 0.7 to about 2.5, preferably from about 0.8 to about 2.2,more preferably from about 0.9 to about 2.0 is attained. The preparationof the alkali cellulose and the reaction of the alkali cellulose withthe alkyl halogenide can be carried out in the same reactor or in twoseparate reactors. The alkyl halogenide is optionally added to thecellulose before or as early as the alkali metal hydroxide, butpreferably it is added after at least a part of the cellulose hasreacted to alkali cellulose. Useful alkyl halogenides generally containfrom 1 to 4 carbon atoms, preferably from 1 to 3 carbon atoms, morepreferably 1 or 2 carbon atoms. Bromides and particularly chlorides arepreferred. Ethyl chloride and particularly methyl chloride are the mostpreferred alkyl halogenides. Generally the molar ratio between the alkylhalogenide and the cellulose, calculated on the basis of ananhydro-D-glucose unit, is from 0.1 to 15.0:1, preferably from 1.0 to10.0:1. Generally step b) is carried out in the presence of an alkalimetal hydroxide such that the molar ratio between the alkali metalhydroxide and the cellulose, calculated on the basis of ananhydro-D-glucose unit, is from 0.3 to 6.5:1, preferably from 1.0 to5.0:1. Preferably, step b) is carried out in the presence of a liquidsuspending agent, such as water or an organic solvent, preferably anstraight-chain or cyclic ether, such as dimethyl ether, ethylene glycolmonoalkyl ether, ethylene glycol dialkyl ether, dioxane ortetrahydrofurane; a C₁–C₆ alkanol, such as isopropanol, ortert.-butanol; a C₁–C₄-alkoxy-(C₁–C₆)-alkanol, or an aromatic oraliphatic hydrocarbon, such as toluene, xylene, hexane or heptane.Preferably the weight ratio between the liquid suspending agent and thecellulose is from 0.1 to 15.0:1, more preferably from 0.2 to 10.0:1.Step b) is generally carried out at a temperature of from 25 to 120° C.,preferably from 40 to 110° C. and a pressure of from 5 to 25 bar (500 to2500 kPa), preferably from 10 to 20 bar (1000 to 2000 kPa). The reactionis generally complete within 30 to 480 minutes.

In step c) of the process of the present invention excess alkylhalogenide is removed from the reaction mixture. By the term “excessalkyl halogenide is removed” is meant that substantially all alkylhalogenide that has not reacted with alkali cellulose in step b) isremoved from the product mixture in step c). Such removal can be carriedout in a generally known way, such as filtration, centrifugation,distillation, or evaporation at reduced pressure. Alkyl halogenideshould be removed to such an extent that substantially no alkylhalogenide is left in the reaction mixture that is able to react in thesubsequent reaction step d). In this reaction mixture the molar ratiobetween the alkyl halogenide and the cellulose used in step a),calculated on the basis of an anhydro-D-glucose unit, is generally lessthan 0.007:1, preferably less than 0.002:1, most preferably less than0.001:1.

After removal of the alkyl halogenide, the alkylated cellulose can bereacted with an alkylene oxide in the subsequent step d) withouttemporary interruption of the process or without isolation of thealkylated cellulose. Alternatively, the reaction may be temporarilyinterrupted and proceeded at a later stage. In the latter case thealkylated cellulose can be isolated in a known way. Advantageously, thereaction mass is suspended in hot water. Preferably, excess alkali metalhydroxide is removed prior to step d) in a known way. Removal of excessalkali metal hydroxide can partially be achieved by use of aqueouswashing liquor. Preferably, a neutralizing agent, such as a mineralacid, carbonic acid or acid anhydride, for example nitric acid,hydrochloric acid, acetic acid, citric acid, or carbon dioxide is addedto the washing liquor to neutralize excess alkali metal hydroxide. Mostpreferably, acetic acid is used for the neutralization. The alkylcellulose ether can be removed from the reaction mixture and washed in aknown manner, for example by filtration or centrifugation. A part orsubstantially all of the alkali metal halide that has been produced as aby-product in step b) may be removed during this washing procedure.Optionally, the alkyl cellulose ether can be dried and comminuted in aknown manner. Drying is for example carried out by a plate dryer, afluid bed dryer, or a milling/drying device. Comminution can be carriedout in a known mill to a particle size of preferably less than 0.5 mm,more preferably less than 0.3 mm.

In step d) of the process of the present invention the alkyl celluloseether produced in step c) is reacted with an alkylene oxide in a liquidsuspending agent to produce an alkyl hydroxyalkyl cellulose ether. Stepd) can be carried out in the same reactor as steps a) and b).Preferably, step d) is carried out in a different reactor than steps a)and b). If excess alkali metal hydroxide has been removed in the processstep c), a base is added to the alkyl cellulose ether obtained in stepc). A catalytic amount of the base is sufficient. The base is preferablyan alkali metal hydroxide, more preferably sodium hydroxide, but otherbases, such as quaternary ammonium bases, are also useful. The alkalimetal hydroxide can be used in a solid form or in the form of an aqueoussolution. When an aqueous solution is used, its alkali metal hydroxideconcentration generally is from 15 to 70 percent, preferably from 45 to55 percent, based on the total weight of the aqueous solution. Generallythe molar ratio between the alkali metal hydroxide and the celluloseused in step a), calculated on the basis of an anhydro-D-glucose unit,is from 0.1 to 1.5:1, preferably from 0.1 to 1.0:1. Preferred alkyleneoxides contain from 2 to 4 carbon atoms. Particularly preferred isethylene oxide or, most preferred, propylene oxide. Generally the molarratio between the alkylene oxide and the cellulose used in step a),calculated on the basis of an anhydro-D-glucose unit, is from 0.05 to20:1, preferably from 0.1 to 10:1.

Step d) is carried out in the presence of a liquid suspending agent,generally an organic solvent which is optionally mixed with water. Auseful organic solvent is for example an straight-chain or cyclic ether,such as dimethyl ether, ethylene glycol monoalkyl ether, ethylene glycoldiethyl ether, dioxane or tetrahydrofurane; a C₁–C₆ alkanol, such asisopropanol or tert.-butanol; a C₁–C₄-alkoxy-(C₁–C₆)-alkanol, or anaromatic or aliphatic hydrocarbon, such as toluene, xylene, hexane orheptane. Blends of organic solvents are also useful. It has beensurprisingly found that the choice of the liquid suspending agent has agreat influence on the swelling of the produced alkyl hydroxyalkylcellulose ether in the reaction mixture. By proper selection of thesuspending agent, the swelling of the alkyl hydroxyalkyl cellulose ethercan be reduced, the suspending agent can be easily removed from thealkyl hydroxyalkyl cellulose ether and the purification of the alkylhydroxyalkyl cellulose ether is substantially facilitated. It has beenfound that the hydroxyalkylation reaction is facilitated and swelling ofthe produced alkyl hydroxyalkyl cellulose ether can be minimized if theliquid suspending agent used in step d) is a solvent or solvent mixturewhich has the following properties i) unpolar character, ii) polarcharacter, and iii) the capability for the formation of H-bonds,preferably in a well balanced manner. These properties can be providedby the employment of three different solvents in a mixture wherein eachof them contributes one of the desired properties, for example heptaneproviding unpolar character, acetone providing polar character, andwater providing the capability to form H-bonds. These balancedproperties can also be achieved by two solvents in a mixture, forexample hexane providing the unpolar character, and 2-propanol providingthe polar and H-bond character. These balanced properties can also bereached in a single solvent providing all three properties, such as2-ethyl-hexanol.

The reaction in step d) is preferable carried out in an alcohol or in asolvent mixture comprising i) an aromatic or aliphatic hydrocarbon, suchas toluene, hexane or heptane, and ii) an alcohol, such as a C₁–C₆alkanol, for example isopropanol, or tert.-butanol or aC₁–C₄-alkoxy-(C₁–C₆)-alkanol, such as methoxy-propanol. The alcohol ispreferably tert-butanol or isopropanol. More preferably, reaction stepd) is carried out in isopropanol or, most preferably, in a blend ofisopropanol and n-hexane. A blend preferably comprises i) from 10 to 90,more preferably from 20 to 80, most preferably from 40 to 60 weightpercent of an alcohol, such as isopropanol, and from 90 to 10, morepreferably from 80 to 20, most preferably from 60 to 40 weight percentof an aromatic or aliphatic hydrocarbon, such as n-hexane.

It has surprisingly been found that the hydroxyalkylation reaction isfacilitated and swelling of the produced alkyl hydroxyalkyl celluloseether can be minimized if the liquid suspending agent used in step d) isa solvent or solvent mixture which has a total solubility parameterδ_(T) of from 10 to 28 MPa^(1/2), preferably from 1 to 26 MPa^(1/2),most preferably from 15 to 24 MPa^(1/2).

The total solubility parameter is defined by the following equation (I)δ_(T)=(δ_(D) ²+δ_(P) ²+δ_(H) ²)^(1/2) MPa ^(1/2)  (I)wherein

-   δ_(D) is the partial solubility parameter for non-polar interaction,-   δ_(p) is the partial solubility parameter for polar interaction, and-   δ_(H) is the partial solubility parameter for hydrogen bonding.    δ_(D), δ_(p), δ_(H) and δ_(T) are known as Hansen solubility    parameters, see C. M. Hansen, Ind. Eng. Prod. Res. Develop. 8, 1,    2–11 (1969) and C. M. Hansen, Hansen solubility parameters, CRC    Press, Boca Raton, Fla. (1999). Both the partial solubility    parameters δ_(D), δ_(p), and δ_(H) and the total solubility    parameters δ_(T) are listed in the literature for a great number of    solvents, see A. F. M. Barton, Handbook of solubility parameters and    other cohesion parameters, CRC Press Inc, Boca Raton, Fla. (1991).

The determination of solubility parameters of polymers includingcellulose ethers, such as hydroxypropyl methyl cellulose ethers andhydroxyethyl methyl cellulose ethers is known, see W. L. Archer, Ind.Eng. Chem. Res. 30, 2292–8 (1991). Cellulose ethers with alkyl and/orhydroxyalkyl groups typically exhibit a total solubility parameter δ_(T)of from 20 to 31 MPa^(1/2). Water-soluble cellulose ethers, such ashydroxyethyl cellulose ethers, hydroxyethyl methyl cellulose ethers orhydroxypropyl methyl cellulose ethers, generally have a δ_(T) of from 28to 31 MPa^(1/2), see W. L. Archer, Drug Dev. Ind. Pharm. 18,599–616(1992). Water has a δ_(T) of 31.7 Mpa^(1/2).

Surprisingly, it has also been found that the hydroxyalkylation reactionis facilitated and swelling of the produced alkyl hydroxyalkyl celluloseether can be minimized if step d) is carried out in a suspending agentsuch that the radius of interaction ^(ij)R is at least 6.0 MPa^(1/2),preferably at least 8.0 MPa^(1/2), and most preferably at least 10.0MPa^(1/2). Preferably, ^(ij)R is not more than 35 MPa^(1/2), morepreferably not more than 30 MPa^(1/2), most preferably not more than 25MPa^(1/2). ^(ij)R is calculated based on the total solubility parameterδ_(T) of the alkyl hydroxyalkyl cellulose ether to be prepared and δ_(T)of the suspending agent according to the following equation II:^(ij)R=(4(^(i)δ_(D)−^(j)δ_(D))²+(^(i)δ_(p)−^(j)δ_(p))²+(^(i)δ_(H)−^(j)δ_(H))²)^(1/2)  (II)In this equation, the “j” terms correspond to the parameters of thesolute, that means the cellulose ether, and the “i” terms to theparameters of the solvent, that means of the suspending agent.

Solubility parameters of solvent mixtures can be calculated from thefollowing equation III, where Φ is the volume fraction of a component inmixture, this means that Φ₁, Φ₂, etc. is the volume fraction of thefirst, second, etc. component of the mixture. δ_(T1), δ_(T2), etc. isthe total solubility parameter of the first, second, etc. component ofthe mixture.δ_(T) (mixture)=Φ₁δ_(T1)+Φ₂δ_(T2)+  (III).

Preferably the weight ratio between the alkyl cellulose ether and theliquid suspending agent is from 0.1 to 30:1, more preferably from 1 to20:1, most preferably from 2 to 15:1. Preferably the weight ratiobetween the alkylene oxide and the liquid suspending agent is from 0.001to 1:1, more preferably from 0.01 to 1:1, most preferably from 0.01 to0.5:1. Step d) is generally carried out at a temperature of from 50 to110° C., preferably from 70 to 100° C. The pressure in reaction step d)generally is from 1 to 12 bar (100 to 1200 kPa), preferably from 1 to 10bar (100 to 1000 kPa), more preferably from 1 to 7 bar (100 to 700 kPa).If an inert gas is used in the reaction step d), the pressure may behigher. If steps b) and d) are carried out in separate reactors, thepressure rating in the reactor for step d) can be lower than in reactionstep b), which is an advantage of the process of the present invention.The alkylene oxide is generally reacted with the alkyl cellulose etheruntil an alkyl hydroxyalkyl cellulose ether with an M.S. (hydroxyalkyl)value of at least about 0.03, preferably at least about 0.05, morepreferably at least about 0.08 and up to about 2.0, preferably up toabout 1.5, more preferably up to about 1.2 is attained. The reaction isgenerally complete within 30 to 480 minutes, preferably within 60 to 360minutes.

The resulting product mixture can be processed in a known manner.Preferably, an above-mentioned acid is added to the product mixture toneutralize the base which is generally present in the mixture. The alkylhydroxyalkyl cellulose ether can be freed from the bulk of liquidcomponents in a separating device, such as a filter or a centrifuge. Theseparated alkyl hydroxyalkyl cellulose ether is then preferably washed 1to 5 times with a washing liquor, preferably with the solvent or solventmixture used in step d). Alkali metal halide that has been produced as aby-product in step b) or d) during neutralization can be removed duringan additional washing step with hot water, which is a known procedure inthe preparation of cellulose ethers. The alkyl hydroxyalkyl celluloseether can be dried, comminuted and sieved in a known manner. Drying isfor example carried out by a plate dryer, tube dryer, fluid bed dryer,milling/drying step, or other known technology. Comminution can becarried out in a known mill to an average particle size of preferablyless than 1 mm, more preferably less than 0.5 mm.

The process of the present invention wherein alkali cellulose is reactedwith an alkyl halogenide, non-reacted alkyl halogenide is removed fromthe reaction mixture, and the alkyl cellulose ether is reacted with analkylene oxide has numerous advantages. The formation of by-products canbe well controlled in type and quantity. Furthermore, the by-productscan be easier used for recycling or are better degradable in treatmentfacilities for environmental protection than by-products from knownprocesses wherein the etherifications of the cellulose with an alkylhalogenide and an alkylene oxide are not strictly separated. Forexample, in the reaction step b) generally only an alkanol and a dialkylether are produced as by-products in addition to the alkyl celluloseether and the alkali metal halide. In the case of methylation of thealkali cellulose, only methanol and dimethyl ether are produced asby-products, which can be easily stripped from water that is present inthe reaction or from the washing agent in step b). In step d) of thereaction only the alkylene oxide can form by-products. Glycols are themost common by-products, which are easily biodegradable in wastewatertreatment facilities. In contrast to known processes wherein an alkylhalogenide and an alkylene oxide are present in the same reactionmixture, no alkylated glycols are formed as by-products. In thementioned known processes alkylated glycols are formed which are muchmore difficult to treat by biodegradation than glycols. Moreover, theseparation of the alkylation step b) and the hydroxyalkylation step d)allows a lower pressure in the hydroxyalkylation step.

According to the above-described process novel alkyl hydroxyalkylcellulose ethers are produced which have a D.S.(alkyl) of from about 0.7to about 2.5, preferably from about 0.8 to about 2.2, more preferablyfrom about 1.0 to about 2.0 and an M.S. (hydroxyalkyl) of from about0.03 to about 2.0, preferably from about 0.05 to about 1.5, morepreferably from about 0.08 to about 1.2, and wherein not more than about15 percent, preferably not more than about 10 percent, more preferablynot more than about 5 percent of the hydroxyalkyl groups are capped withan alkyl group. Essentially only hydroxy groups in the anhydroglucoseunits are alkylated by means of an alkyl halogenide. The “D.S.” is thedegree of substitution, that means the average number of substitutedhydroxy groups per anhydro-D-glucose unit; in the case of cellulose itcan be within the range from 0.0 to 3.0. The D.S. (alkyl) designates thedegree of substitution with alkoxy groups. The “M.S.” is the molardegree of substitution, that means the average number of moles of thesubstituting reagent which have been attached by an ether linkage permole of anhydro-D-glucose unit; in the case of cellulose it can be evengreater than 3.0; it is normally used in order to characterize thosesubstituents on the cellulose ether molecule which can be formed bymultiple substitution at an OH group, for example in the case of thehydroxyalkyl substituent because the resulting OH groups of thehydroxyalkyl groups can also be substituted, like the OH groups of thecellulose itself. The M.S. (hydroxyalkyl) designates the average numberof moles of hydroxyalkoxy groups which have been attached by an etherlinkage per mole of anhydro-D-glucose unit. The novel alkyl hydroxyalkylcellulose ethers generally have a viscosity of from 3 to 1,000,000 mPa·s(millipascal-seconds), preferably from 3 to 500,000 mPa·s, morepreferably from 40 to 250,000 mPa·s, measured as a 2 weight percentaqueous solution using an UBBELOHDE viscosimeter at 20° C.

The present invention is further illustrated by the following exampleswhich should not be construed to limit the scope of the presentinvention. All parts and percentages are by weight unless otherwiseindicated. The alkyl and hydroxyalkyl substitutions indicated in theexamples below are measured and calculated according to ASTM D3876. Theviscosities indicated in the examples below are measured and normalizedto a 2 weight percent aqueous solution using an UBBELOHDE viscosimeterat 20° C.

EXAMPLE 1

Ai) Methoxylation

300 g of ground cellulose pulp are charged to a 10 L horizontal steelreactor. The air is carefully replaced by nitrogen. 150 g of dimethylether are added. Then 600 g of 50% aqueous caustic are added for theactivation of the cellulose. After 20 minutes of alkalization at atemperature of 40° C. and a pressure of 5 bar (500 kPa), 380 g ofchloromethane are feed into the reactor under agitation. The temperatureis raised to 80° C. and kept at this level for 5 hours. The pressureranges from 17.5 to 12 bar (1750−1200 kPa). Then the reactor pressure isreduced to normal pressure. The content of the reactor is slurried upwith 4 L of hot water, neutralized with acetic acid, filtered and washedwith additional 4 L of hot water. After drying and milling a methylcellulose ether is obtained which has a methoxyl substitution (MeO) of21.4 percent, which corresponds to a DS(methyl) of 1.24.

Aii) Hydroxypropoxylation

300 g of the methyl cellulose ether produced in the methylation step Ai)are charged to a horizontal steel reactor and the air is carefullyreplaced by nitrogen. The material is then suspended in 3000 g of anitrogen-purged mixture of 87 percent of isopropanol and 13 percent ofwater and padded with nitrogen. 30 g of 50% aqueous caustic are added tothe suspension for the activation of the residual hydroxy groups of themethyl cellulose ether. After 30 min. of alkalization at a temperatureof 25° C. and a pressure of 2.5 bar (250 kPa), 90 g of propylene oxideare charged to the reactor. The temperature is raised to 75° C. and keptat this level for 150 minutes at a pressure of 3.7 bar (370 kPa). Thenthe reactor pressure is reduced to a pressure of 1.5 bar (150 kPa) andthe reactor temperature is kept at 75° C. Then acetic acid is added forthe neutralization of the caustic. The whole reactor content is dumpedinto a steel pressure filter in order to remove the liquid. Since theformed cellulose ether exhibits a significant swelling in theisopropanol/water mixture, filtration is cumbersome. The cellulose etheris treated with several charges of 3 L of acetone in order to displacethe soaked isopropanol/water. The dried cellulose ether is of excellentcolour and marginal salt content. The produced hydroxypropyl methylcellulose ether has a methoxyl substitution (MeO) of 19.5 percent, whichcorresponds to a DS(methyl) of 1.19, and a hydroxypropoxyl (HpO)substitution of 7.4 percent, which corresponds to an MS (hydroxypropyl)of 0.19. The methyl hydroxypropyl cellulose ether has a viscosity of2,000 mPa·s.

Bi) Methoxylation

In a reaction similar to example Ai) 300 g of ground cellulose pulp arereacted with 640 g of 50% aqueous caustic and 720 g of chloromethane for5 hours. A methyl cellulose ether is obtained with 30.2 percent MeO(DS=1.83).

Bii) Hydroxypropoxylation

The methyl cellulose ether is reacted to the hydroxypropyl methylcellulose ether in the subsequent reaction step similar to example Aii).300 g of the methyl cellulose ether is suspended in 3000 g of a nitrogenpurged mixture of 87 percent of isopropanol and 13 percent of water andpadded with nitrogen. Then 30 g of 50% aqueous caustic and 90 g ofpropylene oxide are added. After the cumbersome filtration, washing anddrying as in example Aii), the produced methyl hydroxypropyl celluloseether has 28.8 percent MeO (DS=1.81), 5.3 percent HpO (MS=0.14) and aviscosity of 10,000 mPa·s.

EXAMPLE 2

Ai) Methoxylation

300 g of ground cellulose pulp are charged to a 10 L horizontal steelreactor. The air is carefully replaced by nitrogen and 150 g ofdimethylether are added. Then 600 g of 50% aqueous caustic are added forthe activation of the cellulose. After 20 minutes of alkalization at atemperature of 40° C. and at a pressure of 5 bar (500 kPa), 500 g ofchloromethane are feed into the reactor under agitation. The temperatureis raised to 80° C. and kept at this level for 5 hours. The pressureranges from 17.5 to 12 bar (1750−1200 kPa). Then the reactor pressure isreduced to normal pressure. The content of the reactor is slurried upwith 4 L of hot water, neutralized with acetic acid, filtered and washedwith additional 4 L of hot water. After drying and milling a methylcellulose ether is obtained which has a methoxyl substitution (MeO) of25.0 percent, which corresponds to a DS (methyl) of 1.47.

Aii) Hydroxypropoxylation

300 g of the resulting methyl cellulose ether from example 2Ai) aresuspended in 2300 g of pure isopropanol purged with nitrogen in ahorizontal steel reactor and padded with nitrogen. After the activationof the starting material with 30 g of 50% aqueous caustic during 15minutes at a temperature of 40° C. and a pressure of 2.5 bar (250 kPa),90 g of propylene oxide are charged to the suspension. Thereafter thereactor temperature is raised to 80° C. and kept at this temperature for3.5 hours at a pressure of 3.7 bar (370 kPa). Then the reactor pressureis reduced to a pressure of 1.5 bar (150 kPa) and the reactortemperature is kept at 75° C. The reactor content is neutralized withacetic acid. The reactor content is dumped into a steel pressure filterand the superficial solvent is removed from the product. The resultinghydroxypropyl methyl cellulose ether is still in a swollen formcontaining residual isopropanol and some water. The swelling is not assevere as in Example 1 but treatment of the product with a non-swellingorganic liquid is still advisable. Thus the swollen product is treatedseveral times with charges of 3 L acetone in order to removeisopropanol/water. The dried hydroxypropyl methyl cellulose ether is ofexcellent color and a negligible salt content. The producedhydroxypropyl methyl cellulose ether has 22.9 percent MeO (DS=1.39), 4.6percent HpO (MS=0.12) and a viscosity of 1,400 mPa·s.

Bi) Methoxylation

The first step of the reaction is conducted according to example 1Bi).

Bii) Hydroxyporopoxylation

In the subsequent reaction step similar to example 2Aii) the methylcellulose ether is converted into the hydroxypropyl methyl celluloseether. Thus 300 g of the methyl cellulose ether is suspended in 2300 gof pure isopropanol and reacted with 30 g of 50% aqueous caustic and 90g of propylene oxide. As in Example 2Aii) swelling is significant. Thusthe swollen product is treated several times as described in Example2Aii). The washed and dried hydroxypropyl methyl cellulose etherexhibits 28.8 percent MeO (DS=1.81), 5.3 percent HpO (MS=0.14) and aviscosity of 1,300 mPa·s. The product is of white colour with negligiblesalt content.

EXAMPLE 3

Ai) Methoxylation

300 g of ground cellulose pulp are charged to a 10 L horizontal steelreactor. The air is carefully replaced by nitrogen and 200 g of dimethylether are added. Then 600 g of 50% aqueous caustic are added for theactivation of the cellulose. After 20 min. of alkalization at atemperature of 40° C. and a pressure of 5 bar (500 kPa), 500 g ofchloromethane are feed into the reactor under agitation. The temperatureis raised to 80° C. and kept at this level for 5 hours. The pressureranges from 17.5 to 12 bar (1750−1200 kPa). Then the reactor pressure isreduced to normal pressure. The content of the reactor is slurried upwith 4 L of hot water, neutralized with acetic acid, filtered and washedwith additional 4 L of hot water. After drying and milling a methylcellulose ether is obtained which has a methoxyl substitution (MeO) of24.3 percent, which corresponds to a DS (methyl) of 1.43.

Aii) Hydroxypropoxylation

300 g of the methyl cellulose ether from example 3Ai) are charged into ahorizontal steel reactor and suspended in 4000 g of a nitrogen purgedmixture of 50 percent of isopropanol and 50 percent of n-hexane andpadded with nitrogen. The suspension is treated with 60 g of 50% aqueouscaustic for the activation of the starting material during 20 minutes ata temperature of 34° C. and a pressure of 3.5 bar (350 kPa). Then 50 gof propylene oxide are added and the reactor is kept at 85° C. for 4.5hours at a pressure of 5.25 bar (525 kPa). Then the reactor pressure isreduced to a pressure of 1.5 bar (150 kPa) and the reactor temperatureis kept at 75° C. The reactor content is neutralized with acetic acidand transferred into a steel pressure filter in order to remove thesolvent. The product exhibits marginal swelling. Residual by-productscan be washed out with small quantities of fresh solvent mixture usedfor the reaction. The product is of white color with a negligible saltcontent. After drying and milling, the hydroxypropyl methyl celluloseether has 22.0 percent MeO (DS=1.35), 6.8 percent HpO (MS=0.17) and aviscosity of 1,600 mPa·s.

Bi) Methoxylation

300 g of ground cellulose pulp are charged to a 10 L horizontal steelreactor. The air is carefully replaced by nitrogen and 200 g of dimethylether are added. Then 650 g of 50% aqueous caustic are added for theactivation of the cellulose. After 20 minutes of alkalization at atemperature of 40° C. and at a pressure of 5 bar (500 kPa), 750 g ofchloromethane are fed into the reactor under agitation. The temperatureis raised to 80° C. and kept at this level for 5 hours. The pressureranges from 17.5 to 12 bar (1750−1200 kPa). Then the reactor pressure isreduced to normal pressure. The content of the reactor is slurried upwith 4 L of hot water, neutralized with acetic acid, filtered and washedwith additional 4 L of hot water. After drying and milling a methylcellulose ether is obtained which has a methoxyl substitution (MeO) of30.2 percent, which corresponds to a DS(methyl) of 1.83.

Bii) Hydroxypropoxylation

300 g of the methyl cellulose ether from example 3Bi) are charged into ahorizontal steel reactor and suspended in 3900 g of a nitrogen purgedmixture of 50 percent of isopropanol and 0.50 percent of n-hexane andpadded with nitrogen. The suspension is treated with 60 g of 50% aqueouscaustic for the activation of the starting material during 20 minutes ata temperature of 34° C. and a pressure of 3 bar (300 kPa). Then 90 g ofpropylene oxide are added and the reactor is kept at 85° C. for 4.5hours at a pressure of 4.2 bar (420 kPa). Then the reactor pressure isreduced to a pressure of 1.5 bar (150 kPa) and the reactor temperatureis kept at 75° C. The reactor content is neutralized with acetic acidand transferred into a steel pressure filter in order to remove thesolvent. The product exhibits marginal swelling. Residual by-productscan be washed out with small quantities of fresh solvent mixture usedfor the reaction. After drying and milling, the hydroxypropyl methylcellulose has 28.6 percent MeO, (DS=1.79), 4.8 percent HpO (MS=0.12) anda viscosity of 42,000 mPa·s. The product is of white color withnegligible salt content.

1. A process for preparing an alkyl hydroxyalkyl cellulose ethercomprising the steps of: a) alkalizing cellulose to obtain alkalicellulose, b) reacting alkali cellulose with an alkyl halogenide untilan alkyl cellulose ether with a D.S.(alkyl) value of from 0.7 to 2.5 isattained, c) removing excess alkyl halogenide from the reaction mixture,and d) reacting the alkyl cellulose ether with an alkylene oxide in aliquid suspending agent to produce an alkyl hydroxyalkyl celluloseether, wherein the weight ratio between the alkylene oxide and theliquid suspending agent is from 0.001:1 to 1:1 and the liquid suspendingagent is a blend comprising from 10 to 90 weight percent of an alcoholand from 90 to 10 weight percent of an aromatic or aliphatichydrocarbon.
 2. The process of claim 1 wherein in step b) alkalicellulose is reacted with an alkyl halogenide until an alkyl celluloseether with a D.S.(alkyl) value of from 0.8 to 2.2 is attained.
 3. Theprocess of claim 1 wherein excess alkali metal hydroxide is removed instep c).
 4. The process of claim 1 wherein in step d) the alkylcellulose ether is reacted with the alkylene oxide until an alkylhydroxyalkyl cellulose ether with an M.S.(hydroxyalkyl) value of from0.03 to 2.0 is attained.
 5. The process of claim 1 wherein step d) iscarried out in a blend of isopropanol and n-hexane as a suspendingagent.
 6. The process of claim 1 wherein the alkyl halogenide in step b)is methyl chloride and the alkylene oxide in step d) is propylene oxide.7. The process of claim 1 wherein the alkyl halogenide in step b) ismethyl chloride and the alkylene oxide in step d) is ethylene oxide. 8.The process of claim 1 wherein steps b) and d) are carried out inseparate reactors.
 9. A process for preparing an alkyl hydroxyalkylcellulose ether comprising the steps of: a) alkalizing cellulose toobtain alkali cellulose, b) reacting alkali cellulose with an alkylhalogenide until an alkyl cellulose ether with a D.S.(alkyl) value offrom 0.7 to 2.5 is attained, c) removing excess alkyl halogenide andexcess alkali metal hydroxide from the reaction mixture, and d) reactingthe alkyl cellulose ether with an alkylene oxide in a liquid suspendingagent to produce an alkyl hydroxyalkyl cellulose ether, wherein theliquid suspending agent is a blend comprising from 10 to 90 weightpercent of an alcohol and from 90 to 10 weight percent of an aromatic oraliphatic hydrocarbon.
 10. The process of claim 9 wherein the suspendingagent in step d) is a blend of isonropanol and n-hexane.